The invention is related to a method for temperature compensation of gas analyzer equipment for transient error caused by temperature changes.
The invention also concerns an apparatus with temperature compensation of transient error in gas analyzer equipment caused by temperature changes.
The invention further concerns a method and an apparatus for improving the measurement accuracy of a gas analyzer based on thermal detection of infrared radiation, in particular under changes in the operating temperature of the analyzer.
The invention is particularly suited to applications in which the infrared radiation intensity is determined by a direct or indirect measurement of the temperature difference produced between the sensor element detecting the impinging radiation and a reference element. Such a detector has the inherent shortcoming that a change in the ambient temperature disturbs the internal thermal balance of the detector, and resultingly, the output signal will contain a transient error which degrades the accuracy of the gas analyzer during the temperature change. The thermal detector used in the measurement may advantageously be a thermopile detector.
The method according to the invention is suited for use in gas analyzers designed to perform either the identification or measurement of concentration, or alternatively, both the identification and measurement of concentration, of at least one component of a gas sample. Such a gas analyzer can be used for, e.g., monitoring the composition of the airway gases of a patient anesthetized for the duration of an operation, whereby the gases to be determined can include at least carbon dioxide (CO.sub.2), nitrous oxide (N.sub.2 O) as well as at least one anesthetic agent.
The thermopile detector measures the infrared absorption of gas introduced to a sampling chamber, after which the concentration of the desired gas component is determined from the measured absorption.
Thermopile detectors are used in gas analyzers among other things owing to their capability of DC measurement which facilitates a cost-effective construction of the measurement system. The wavelength range of thermopile detectors is suitable for infrared measurements, since the absorption bands centering about the 10 .mu.m wavelength fall within the spectral sensitivity wavelength range of the detector. Moreover, thermopile detectors have a high sensitivity and good linearity.
A characteristic of the thermopile detector is that with a change of its external housing temperature after, e.g., a cold start-up of the analyzer or due to an ambient temperature change, the detector output will contain a transient error which degrades measurement accuracy over the duration of the transient state.
With reference to U.S. Pat. No. 4,423,739 (Passaro), the gas analyzer output signal can be compensated for thermal drift by means of a chopper which intermittently blocks the radiation path from the lamp to the detector, thus permitting measurement of detector output signal offset which is then subtracted from the detector output signal when the radiation is again permitted to impinge on the detector. The moving mechanical components of..the chopper make the analyzer not only large and costly, but lower its operational reliability.
With reference to U.S. Pat. No. 3,745,349 (Liston), a gas analyzer can also be stabilized by way of modulating the radiant output power of its infrared source. However, such modulation is possible only at a relatively low frequency, which thus limits the response speed of the gas analyzer.
As is disclosed in the PCT publication WO91/18279 (Apperson), the error related to the temperature change can be avoided by thermostatting the operating temperature of the infrared detectors. However, as the thermopile is substantially disturbed even by the slowest changes in its operating temperature, the temperature control system must have a slow response with low drift. Such a slow response results in retarded start-up of the analyzer. Designing the temperature control system for low drift makes is costly. Heating/cooling arrangements in turn increase the power consumption and size of the analyzer.
With reference to U.S. Pat. No. 4,772,790 (Aldridge), a gas analyzer based on a thermopile detector is disclosed in which a quadruple detector array encapsulated in a single package is subjected to infrared radiation so that the radiation components passed through the sample gas cell and attenuated by the gases under measurement are arranged to impinge on three measurement sensor elements, while such a wavelength of the radiation passed through the sample gas cell that is free from absorptive attenuation by the gases under measurement is arranged to impinge on one sensor element acting as the reference element. Then, the thermal drift caused temperature change can be compensated by subtracting the output signal of the reference element from the output signals of the measurement sensor elements. The system is hampered by the need for a band pass filter centered at the reference wavelength which contributes to the higher cost of the analyzer. Moreover, the requirements set to the measurement electronics are tightened substantially, because the output voltage of each sensor element in the quadruple detector structure is maximally one-fourth of the output voltage of an equivalent-size single sensor element package.
Replacing the filter centered at the reference wavelength, one of the sensor elements of the thermopile package can be covered with a foil opaque to infrared radiation such as a metal foil. Then, the temperature compensation can be made by subtracting the output voltage of the covered sensor element from the output voltage of the measurement sensor element. This approach is hampered by the need for the extra sensor and that the operating temperature differences of the sensor elements with their different temperature drift characteristics cause incorrect compensation.
With reference to WO patent publication 91/03204 (Yelderman), the output signal error caused by the temperature drift of the thermopile detector is compensated by performing the radiation absorption measurement of the gas of interest by means of two thermopile detectors encapsulated in a single package, whereby in front of one of the sensor elements is placed a filter with an optical attenuation of, e.g., 50% to the radiation wavelength being measured. Then, the temperature change affects both detectors in the same manner, whereby the effect of the temperature-dependent transient error is compensated when the difference of the detector output voltages is formed. Owing to the radiation-attenuating filter placed in front of one sensor element, the differential output voltage of the sensor elements is still proportional to the incident radiation intensity. The method is hampered by the need for a dual detector and an auxiliary filter having a specified attenuation characteristic for the incident radiation. Due to the use of a dual detector and formation of a difference of the detector output voltages, the output voltage of such a compensated detector is maximally one-fourth of the output voltage of an equivalent-size single sensor element package.
Sensing of the internal temperature of thermopile detectors used in gas analyzers has been employed in some applications. However, such simple temperature information has not proven sufficient for the compensation of thermal drift under a transient situation or change in ambient temperature.